Mobile communication systems, such as those used for cellular telephone communication, divide the spectrum into a multiplicity of individual signaling channels or frequency bands. Particular channels are allocated to individual users as they access the system. Each user's communication path is routed through the system through the channel allocated to that user. Signals broadcast by the system must be carefully regulated so that they remain within the channels allocated to the various users. Signals “out of band” can spill over from one channel to another, causing unacceptable interference with communications in the other channels. In order to increase the data transfer in such channels linear modulations like QAM, 8-PSK and others containing amplitude modulation are utilized in contrary to older systems using constant amplitude phase or frequency modulation. The new 3G systems will need multi-carrier amplifiers. Those new modulations require high linearity amplifiers and up-converters not to cause interference to other channels in the cellular system. Although combining of a plurality of carriers of any modulation into a single RF-power amplifier (MCPA) means that the amplifier will require very high demand on linearity in order to avoid spectral re-growth spreading RF-power into regions of the spectrum not appearing in the input signal.
Normal RE-power linearization techniques are utilizing the known Feed-Forward technique (FF) and variants thereof. In order to improve the performance of the concept various attempts of improving the FF-architecture by pre-distorting the signal to the main power amplifier are utilized. This is done to reduce the distortion in the main amplifier before applying the correcting signals in the feed forward loop thus achieving better efficiencies and less correcting needed in the FF-loop. Examples of such patents are WO97/37427, WO99/23756, WO99/45640 and WO99/45638 which shows a general increase of analogue complexity of generating the pre-distortion signals to the Main Power amplifier in a Feed-Forward Application or only using pre-distortion linearization of an RE-amplifier without the feed-forward loop for less demanding applications. With the semiconductor technology improving in both DSP- and ADC- and DAC- techniques a strive has been taken of doing the pre-distortion in the digital domain instead of the analogue. Various patents on digital predistortion have been filed. First the digital predistortion patents were covering linear modulation single carrier amplifier improvements. References to be mentioned here are U.S. Pat. No. 4,291,277, U.S. Pat. No. 5,049,832 et al. Technical articles like James Carver-IEEE Transactions on Vehicular technology, Vol. 39 No. 4, November 1990: “Amplifier Linearization using Digital Pre-distorter with fast adaptation and Low Memory requirements” and Andrew S. Wright and Willem Durtler, IEEE Transactions on Vehicular technology, Vol. 41, No 4, November 1992:“Experimental Performance of an adaptive Digital Linearized Power Amplifier” gives good insight in the preceding history for the evolution of Digital Pre-distortlon. In FIG. 1 is illustrated a digital pre-distorter outline as described by Carvers et al.
RF-power multi-carrier power amplifiers (MCPA) require very high demand on linearity in order to avoid spectral re-growth spreading RF-power into regions of the spectrum not appearing in the input signal. In analogy with the known analogue feed-forward technique, different patents of digital pre-distortion and post-distortion implementations are disclosed in for instance patent documents WO97/30521, WO98/51005, U.S. Pat. No. 5,923,712 by Leyondecker and WO98/12800.
FIG. 2 shows a basic outline for a digital pre-distortion (DPD) application applied to a wireless system. However, DPD can be applied to other systems needing digital linearization. The mentioned patents deals with the implementation of the so called digital real time circuit and then in smaller content with the calculation routines (algorithms) used in the DSP for up-dating look-up tables and other steering parameters. A practical design must take care of both the hardware and the software for the ease of the practical implementation.
All mentioned patents relies on the basic structure illustrated in FIG. 1 with some functional additions to handle and compensate for more than the basic gain and phase non-linearity transfer function, that a real physical device has. The digital model for the non-linear device like an amplifier (PA) must incorporate models containing more dimensions of data taking account for the so called “Memory effects”. By integration of the input signal over a certain time a measure of the input signal Peak to Average signal level is made. This is then utilized to create tables describing the device performance dependence not only dependent of the actual input signal strength. The patent document WO98/12800 from Spectrian describes one way of from measured amplifier performance by use of a so called “leaky integrator” get information of the moving average magnitude of the signal and from that create a function to describe the performance of the amplifier combined into one table. The Spectrian patent used the signal magnitude as input to the “leaky integrator”, which is basically wrong as the claims are for power dependence. The “leaky integrator” shall work on the squared magnitude representing the signal power instead. The above mentioned patent as well as U.S. Pat. No. 5,949,283 and U.S. Pat. No. 5,959,500 are different implementations of how to create tables from observations of the amplifier output signal. The observations are used to create tables to pre-distorted input signals to the amplifier in order to improve the distortion at the output of the amplifier. By adding complexity in the pre-distorters the look-up table (LUT) dimension are often increased drastically. The patents also deals with the scenarios of using the created look-up tables to create signals to be used as a post-distortion that is subtracted from the main amplifier by another amplifier up-converter at the output of the main amplifier. This adds complexity to the solutions.
The present invention only applies to the design of the digital parts needed to make distortion cancellation of a non-linear device like a RF-power amplifier (PA) and the algorithms for achieving the results. The power amplifier is considered to be the non-linear device for the rest of this paper. Different outlines and patents have been granted for this issue. Worth mentioning is the following. The results achieved in those patents are that very large multidimensional memory size is needed and the algorithms for calculating the needed memory contents are unclear. The available distortion cancellation in applying these patents are also unknown as the structures and algorithms does contaminate different PA performances of a real device like phase delay, power dependence and bias dependence in the same function blocks of the implemented digital block diagrams. U.S. Pat. No. 5,923,712 describes a method of tables containing extracted several weighting taps in some peculiar manner combining both power and magnitude samples with different delays to decide some average performance. The result is combined with direct inverse pre-distortion modeling. FIG. 8 in U.S. Pat. No. 5,923,712 shows how complex the implementation becomes for a practical case if memory predictions are going to be used. Multidimensional tables are also implemented for power dependence predictions disclosed in other patents.
The basic for all these patents are that the compensating gain calculations to be put into the LUT is done by a direct inverse division of the observed RF-power output signal and a time delayed (adjusted) input signal. There are a lot of special designed algorithms needed to be applicable to each particular patent for improving the basic failures from direct inverse calculations and the particular outlines used like signal noise sensitivity reduction and algorithm convergence.
Recently published patents: US2001/050592 A1 and US2002/0008578 A1 are the most promising approach based on Volterra analysis and clever mathematics of solving the Volterra kernels (See references [1] and [2]). The result is multiple Look up tables where each table dimension is read from a different steering parameter and the result is added and applied to coefficients of a FIR-filter to handle memory effects.
Thus there is still a need for a simple and efficient method for providing RF-power amplifier distortion minimizing (i.e.; linearization or pre-distortion). Therefore the present invention does not perform direct inverse calculations as outlined above and which will be explained in the description of the present new procedure.